Generally, an optical fiber used for usage such as communication has wavelength dependency in a longitudinal direction. For this reason, in particular, in order to transmit a signal accurately by using an optical fiber used for a channel whose line length is long, it is necessary to take this wavelength dependency into consideration. What expresses this wavelength dependency is a “distributed map.” This distributed map shows the relationship between the distance in the longitudinal direction and the wavelength dispersion of an optical fiber. As prior art for determining a distributed map, an apparatus disclosed in Japanese Patent Laid-Open No. 10-83006 is known.
The operation principle of this apparatus is to detect Stokes light or anti-Stokes light out of the return light which is obtained by entering the pulsed light, in which two kinds of rays which have wavelengths λ1 and λ2 are included, into an optical fiber, and to obtain wavelength dispersion D on the basis of a frequency of this detection output. By using this method, it becomes possible to produce correctly and easily the distributed map showing the relationship between the distance z along the longitudinal direction of the optical fiber and the wavelength dispersion D in its place by a method like Optical Time Domain Reflectometry (OTDR).
By the way, since the calculation of wavelength dispersion D using the apparatus disclosed in the gazette mentioned above calculates on the basis of the frequency of intensity amplitude of the detected light, only the absolute value can be found, but there is a problem that a sign is not known. However, in actually using an optical fiber as a channel, the sign of wavelength dispersion D is also required. For example, in designing the channel, which is a grade where wavelength dispersion can be disregarded mostly as a whole, by connecting two or more kinds of optical fibers which have predetermined wavelength dispersion, it becomes important whether the wavelength dispersion has a positive value or it has a negative value. Since the sign of wavelength dispersion is not found by the method disclosed in the gazette mentioned above, the sign of wavelength dispersion is measured by another method, and hence, there is a problem that determination requires troubles and time since it is necessary to change apparatuses connecting optical fibers or to prepare an apparatus determining the sign of wavelength dispersion in addition to the distributed map.